Subjective experience exists, and even if we can reject free will, the perception of free will is undeniable. Because we have a subjective experience (i.e., we have a mind) and we have the will to create its contents, we are able to create purpose.
There is no single universal mind and, subjective experiences are unique to each human being. Therefore, each human being creates their own purpose. There is no single purpose everyone must follow and there is no destiny. We seem to enter life with a tabula rasa, and we create what we paint on it (do we enter the world with a tabula rasa).
When we get to the point where we want to create purpose, there are ways to create our individual purpose.